FIELD OF THE INVENTION
The invention relates generally to data storage devices wherein data access is achieved by positioning a transducer relative to a storage medium, positioning being controlled by a servo system in response to positional information. More particularly it relates to an improved disk drive apparatus and method for writing positional information to the medium.
Increased levels of storage capacity in storage devices such as hard disk drives (optical or magnetic, for example) and storage media (removable disk or removable tape drives, for example) are a direct result of the higher track densities possible with voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magnetoresistive (MR) head technology. Head positioning is accurately controlled using positional information stored on the disk itself, as in dedicated and embedded servo architectures.
Conventional servo-patterns, e.g. in an "embedded servo" disk drive, typically comprise short bursts of a constant frequency signal, very precisely offset to either side of a data track's center line. The bursts precede data regions of the track and are used to align a head with respect to the track center. Staying on track center is required during both reading and writing for accurate data storage and retrieval. Since there can be, for example, sixty or more data regions per track, that same number of servo data areas are preferably distributed around a track to provide means for a head to follow the track's center line as the disk rotates, even when the track is out of round, e.g., as a result of spindle wobble, disk slip and/or thermal expansion. As technology advances to provide smaller disk drives and increased track densities, the accurate placement of servo data must also increase proportionately.
Servo-data are conventionally written by costly dedicated servowriting equipment external to the disk drive equipped with large granite blocks for stably supporting the drive and quieting external vibrational effects. An auxiliary clock head is inserted onto the surface of the recording disk to write a reference timing pattern, and an external head/arm assembly is used to precisely position the transducer. The positioner includes a very accurate lead screw and a laser displacement measurement device for positional feedback. Servotracks are written on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servotracks.
A disadvantage of servo writers such as those described is that they require a clean room environment, as the disk and heads will be exposed to the environment to allow the access of the external head and actuator. Additionally, it is becoming more and more difficult for such servowriters to invade the internal environment of a HDA for servowriting because the HDAs themselves are exceedingly small and depend on their covers and castings to be in place for proper operation. Some HDAs, for instance, are the size and thickness of a plastic credit card.
In view of these challenges, a disk drive able to perform self-servo writing would be tremendously advantageous. However, this approach presents a new set of challenges. Specifically, self-servowriting systems are more prone to mechanical disturbances. Moreover, because of the interdependency of propagation tracks in self-servowriting, track shape errors introduced by mechanical disturbances and other factors may be amplified from one track to the next when writing the propagation tracks. Thus a self-servowriting system must be able to write servopatterns with a high degree of accuracy to meet the stringent requirements of high density disk drives.
Servopatterns consist of bursts of transitions located at intervals around the disk surface. In self-propagation, the radial position signal that is used to servo-control the actuator is derived from measurements of the readback amplitude of patterns that were written during a previous step of the servowrite process. That is, the burst edges of a written track comprise a set of points defining a track shape that the servo controller will attempt to follow when writing the next track. Thus, errors in the transducer position during burst writing appear as distortions away from a desired circular track shape. The servo controller causes the actuator to follow the resulting non-circular trajectory in a next burst writing step, so that the new bursts are written at locations reflecting (via the closed-loop response of the servo loop) the errors present in the preceding step, as well as in the present step. Consequently, each step in the process carries a "memory" of all preceding track shape errors. This "memory" depends on the particular closed-loop response of the servo loop.
Effects that result in track shape errors include, for example, random mechanical motion and modulation in the width of the written track that results from variations in the properties of the recording medium or in the flying height of the transducer. Uncontrolled growth of such errors can lead to excessive track non-circularity. In some cases, error compounding may even lead to exponential growth of errors, exceeding all error margins and causing the self-propagation process to fail. Consequently, self-servowriting systems must provide a means for accurately writing servopatterns while controlling the propagation of track shape errors.
One self-servo writing method is disclosed in U.S. Pat. No. 4,414,589 to Oliver et al., which teaches optimization of track spacing. Head positioning is achieved in the following manner. First, one of the moving read/write heads is positioned at a first stop limit in the range of movement of the positioning means. The head is used to write a first reference track. A predetermined percentage of amplitude reduction, X %, is selected that empirically corresponds to the desired average track density. The moving head reads the first reference track and is displaced away from the first stop limit until the amplitude of the signal from the first reference track is reduced to X % of its original amplitude. A second reference track is then written by the head at the new location, read, and the head is again displaced in the same direction until the amplitude of signal from the second reference track is reduced to X % of its original value. The process is continued until the disc is filled with reference tracks. The average track density is checked to insure that it is within a predetermined acceptable range of the desired average track density. If the average track density is too high or too low, the disk is erased, the X % value is appropriately lowered or increased, and the process is repeated. If the average track density is within the predetermined acceptable range, the desired reduction rate X % for a given average track density has been determined and the servo writer may then proceed to the servo writing steps. Thus while Oliver provides a means for positioning the heads, it fails to teach how to limit the growth of errors during the radial propagation.
U.S. Pat. Nos. 4,912,576 to Janz and 5,448,429 to Cribbs et al. describe methods for writing a servo-pattern with a disk drive's own pair of transducers. Three types of servo-patterns are used to generate three-phase signals that provide a difference signal having a slope directly proportional to velocity. Janz observes that the signal level from a transducer is a measure of its alignment with a particular pattern recorded on the disk. For example, if the flux gap sweeps only forty percent of a pattern, then the read voltage will be forty percent of the voltage maximum obtainable when the transducer is aligned dead-center with the pattern. Janz uses this phenomenon to position the heads by straddling two of three offset and staggered patterns along a centerline path intended for data tracks. In a preferred process, Janz describes a dedicated servo architecture wherein one side of a disk is reserved for servo and the other side for data. The disk drive includes two transducers on opposite surfaces that share a common actuator. To format an erased disk for data initialization, a first phase servo is written on the servo side at an outer edge. The transducers are then moved-in radially one half of a track, as indicated by the first phase servotrack amplitude, and a first data-track is recorded on the data side. The transducers are again moved radially inward one half of a track, this time as indicated by the first data-track amplitude, and a second phase servotrack is recorded on the servo side. This sequence is repeated until both surfaces are entirely written. If too few or too many tracks are written, the disk is reformatted with a slight adjustment to the step width, as determined by the track count. Once the disk drive has been formatted with an entire compliment of properly spaced servotracks, the data-tracks are erased in preparation for receiving user data. Unfortunately, the method described by Janz requires a dedicated disk surface for servotracks and two heads working in tandem. Moreover, transducer flying height variations, spindle runout, and media inconsistencies can corrupt radial position determinations that rely on a simple reading of off-track read signal amplitudes. Prior art methods do not address these issues and are therefore inadequate for high performance disk drives applications.
Cribbs et al. teaches a hard disk drive system with self-servowriting capability comprising a rotating recording disk, transducer in communication with the disk surface, servo-actuator means for radially sweeping the transducer over the surface, a variable gain read amplifier (VGA) coupled to the transducer, an analog to digital converter (ADC) coupled to the VGA, an erase frequency oscillator coupled to the transducer for DC erasing of the disk surface, a memory for storing digital outputs appearing at the ADC, and a controller for signaling the servo-actuator to move to such radial positions that result in transducer read amplitudes that are a percentage of previous read amplitudes represent in the digital memory.
Again, the problem of growth of errors is not specifically addressed by Cribbs et al. The reference notes that track width modulation arising from flying height variations is a source of track shape error that impacts the self-propagation process. A procedure is outlined in which three extra revolutions of the disk are used to smooth the servo error control signals so as to reduce "hunting" and "dithering" of the servo actuator before each step of writing propagation bursts. It is unlikely that track width modulation large enough to result in excessive "hunting" could occur within any one step of burst writing, particularly since width modulation is a secondary effect compared to on-track readback modulation, and since a preliminary step in the Cribbs process is to reject all disk files having excessive on-track modulation. It is more likely that, in accordance with our experiences and detailed analysis, intrinsic track width modulation typically appears at the level of only a few percent of the track width, but through error compounding track noncircularities grow to much larger levels. It is also apparent that a signal discernible in the position error signal of a high gain servo loop is indicative of an underlying track shape error far greater than the error signal itself. This follows from the fact that the position error signal is merely the residual part of the underlying track shape error that the servo loop was unable to follow. Thus, as previously suggested, Cribbs' procedure for adjusting target amplitudes while track following in order to smooth the position error signal is one in which the underlying track shape error appears to be merely hidden, not eliminated.
Even assuming that the smoothing method works for all types of servo loops, which is unlikely, the solution proposed in Cribbs et al. is unattractive because three extra revolutions of the disk are required at each step in the process. Consequently, this approach doubles the servowrite time and raises the implementation cost.
Commonly assigned U.S. patent application Ser. Nos. 08/349,028 and 08/405,261 describe self-servowriting systems which overcome the previously described problems. Head positioning is achieved by first writing a reference track, e.g. with a plurality of propagation bursts, then moving the head to a next position while reading the reference track until it is determined that the amplitude of the readback signal has been reduced by a predetermined amount. The determination is preferably made on a sector-by-sector basis in a two-step process. First, the signal amplitude of each burst is compared with a corresponding normalization value measured in the same circumferential position of the last written track to obtain a propagation burst fractional amplitude. This current value is then compared to a reference value for the sector, and the difference is used as a position error signal (PES) for making corrections to the head position. The PES is also stored for later use. The normalization values are updated for each newly written track in a normalization revolution. Updating for every track has been performed previously because the propagation burst amplitudes from track to track which provide the normalization values tend to vary due, e.g., to fly height variation and modulation of the magnetic properties of the disk or other causes. New reference values are also calculated for each track during the normalization revolution and incorporate the stored PES values and have the effect of reducing track shape error growth. Preferably, the new reference values each comprise a nominal reference value plus a corrective value calculated from the previously stored PES for each sector. In addition, the servo loop is designed to have a closed loop response which causes track shape errors to decay, rather than grow, from one track to the next. A drawback of the proposed scheme is that track-to-track updating of the normalization and reference values adds time to the servowriting process.
Accordingly, what is needed is a self-servowriting disk file which the preceding problems of accuracy associated with self-servowriting while providing acceptable servowriting performance.